The present invention is directed to a method for controlling control channel re-selection by a mobile communication station. More particularly, the invention relates to allowing the re-selection process to be conducted only with respect to those candidate control channels, onto which the mobile station is eligible to camp.
In the wireless communication environment it is known that as a mobile station moves throughout an area it is likely to encounter different cells that are serviced by different base stations. One example of a multi-cell wireless environment is illustrated in FIG. 1. There are a plurality of cells (C1 to C12) and each is served by a separate base station (BS1 to BS12). As a mobile station moves from one cell to another cell it is served by a different base station.
Typically the base stations operate with distinct control channels, voice channels or data channels. It is known that during a communication, such as a call, as a mobile station passes from one cell to another it is handed off from one base station to another according to a certain protocol. However, in an activated state where a call is not in process, while the mobile station is in a cell, it is camped on a control channel, i.e., it is in essence tuned to that channel and prepared to engage in communications of control information over that channel. The mobile station must execute a process for selecting another control channel as the mobile station moves from cell to cell. This is called a re-selection process.
In the wireless environment, a known standard referred to as the IS-136 Standard defines protocols for digital wireless communications involving digital mobile stations. That standard defines a process by which the mobile station can re-select a control channel as it moves among the cells. In that process the mobile station is presumed to be “camped-on” to a first control channel. That control channel then transmits a list of the neighboring control channels, that is the list of control channels for the neighboring cells (or service providers). The mobile station receives the list and stores it. Then in accordance with a prescribed algorithm, the mobile station determines which one of the control channels on the list constitutes the primary candidate for re-selection. This determination is made based on such things as reading the RF (Radio Frequency) level of the various control channels in the list and determining which is the control channel that the mobile station should try to select next. For example, if the mobile station was in cell C2 moving towards cell C1, it may detect that the next strongest control channel among the neighbors of cell C2 originates from base station BS1 and therefore would deem that control channel to be the primary candidate for re-selection.
Further, in accordance with the algorithm described in the IS-136 standard once a primary candidate has been determined the mobile station attempts to re-select to that control channel. If that re-selection attempt is successful, then the mobile station camps onto the new control channel, receives a new list of neighboring control channels and begins the process all over with the new list of neighbors. If the attempt is unsuccessful, then the mobile station either selects the next appropriate candidate channel (that is a secondary candidate channel) or repeats the analysis of the entire list of candidate channels attempting to determine another primary candidate channel for re-selection.
A problem arises in that when the mobile station receives the neighbor list, it may include control channels with which the mobile station is simply incompatible. For example, the list may include one or more channels that are related to private systems and a mobile station may not be permitted access to those private systems. Alternatively, the mobile station may be seeking voice service and the control channels may be directed to data or fax services. Nonetheless, based on the parameters used in the analysis algorithm, these control channels with which the mobile station cannot truly interact are included in the analysis operation and one of these incompatible control channels could be selected as the primary candidate control channel. Naturally when this occurs, the attempt to re-select to that control channel will fail. Thus, the execution of the process has wasted time and resources toward the attempt to re-select an incompatible control channel. Furthermore, under the known algorithms, once the attempt to re-select to the incompatible control channel has failed-that channel is still included in the next cycle of the re-selection process and could be once again selected as the primary candidate control channel. It is conceivable that the mobile station could get caught in an extended loop of determining an incompatible control channel to be the primary candidate control channel and making a failed attempt to re-select to that control channel.
In view of the shortcomings of the known re-selection process, it is desirable to provide further controls in the re-selection process to avoid the attempt to re-select to incompatible control channels. It is also desirable to avoid second attempts to re-select to control channels which are determined to be incompatible with the mobile station during a re-select attempt.